As a function of the state of the parameter to be measured, capacitive sensors have different unknown capacitances, which must be determined for obtaining the indicated parameters. Such capacitive sensors are e.g. pressure sensors, moisture sensors, inclination sensors or the like, which are widely used in measurement technology.
In sensor systems with capacitive sensors the capacitances of the sensors are generally very small and of the order of magnitude of parasitic or stray capacitances of the measurement network, such as line capacitances, connection capacitances of connected ICs, as well as miscellaneous capacitances of electronic parts in the measuring network. With respect to the low capacitances of the capacitive sensors, such stray capacitances are not negligible, because they are also in the picoFarad range of the measuring capacitance and also have a not inconsiderable temperature variation, leading to temperature-dependent measuring errors and in particular to a span error.
In the case of measuring arrangements with capacitive sensors in the high temperature range of several hundred degrees Celsius, in which the measuring and reference capacitance can still be made temperature-stable, they are operated offset via a cable to avoid overheating of the electronics. Said cable, generally a coaxial cable, has a significant inherent capacitance (e.g. 60 pF/m), which is frequently higher than the measuring and reference capacitance. There are significant gains and offset errors, which can rapidly lead to a no longer acceptable range.
It is known that such stray capacitances can be suppressed in that the sensor signal of the inner conductor of the particular coaxial cable is decoupled with an amplifier having a +1 amplification and said signal is applied to the coaxial cable screen. Thus, the coaxial cable screen is at the same potential as the inner conductor. As no potential difference arises, in the ideal case no current flows in the stray capacitor and there is no measured value falsification. However, a disadvantage of this procedure is that several analog amplifiers are required which must have a very good quality, because otherwise as a result of phase shifts in the analog amplifier and the noise thereof additional errors can arise, which can sensitively influence the compensation effect of this method.
Whilst avoiding the aforementioned disadvantages, the problem of the invention is to provide a method and a device for measuring the capacitances of capacitive sensors where, whilst excluding stray capacitances, it is possible at limited cost to obtain high measuring rates during measurements with small operating currents.